Git clone private repo, permission denied

I am trying to clone a private repository owned by another developer. I do not have direct communication with this developer. They sent me a theirusername-id_rsa.pub file and a theirusername-priv.key.ppk file. I understand the ppk file is specific to to Putty ssh client. Can someone provide me with steps on how to clone their repo? I already have git configured with my own account and I think I have to add their ssh key to my ssh/config file or something, but I'm a bit of an ssh noob.

They sent me a theirusername-id_rsa.pub file and a theirusername-priv.key.ppk

First of all, tell them to never do this. The public/private keys are called like that for a reason. It’s against its point to have multiple instances (persons or computers) share the same key. You should use a separate key for every single one, to have a 1-to-1 association between them.

If they can’t add you as a collaborator (which would require a user account for yourself), then they should just add your key to their own profile. I’m emphasizing on “your key”, as you should generate it and send them only the public key. The private one should always remain secret to everybody else.

That being said, when you have a PPK, a PuTTY private key file, you have two options. First would be to use PuTTY’s pageant to load the key file and make Git use PuTTY’s plink as the SSH client. You can do that by setting the GIT_SSH environment variable to the path to plink.exe, e.g. C:\Program Files\PuTTY\plink.exe.

The second option would be to convert the PPK to an OpenSSH key file which the SSH client that comes with Git can use. You can do that by opening the PPK with PuTTYgen and choosing “Conversions/Export OpenSSH key”. You should save the file as C:\Users\<username>\.ssh\id_rsa to make Git use it.